The serrated blade (at least my Spyderco's) can easily be sharpened on a Spyderco Triangle sharpener (along with plain edge knives).

The serrated edge is MUCH better for cutting fiberous materials continuously. The first few cuts are just about the same with a plain edge. Soon the serrated edge shows it's advantage. One of the first Spyderco Mariners cut 6 rolls of fiberglass tape into 10 inch lengths after 3 other good plain edge blades (all longer) wore down on the first two rolls. Not a stunt, we needed the tape to fair a long cable we were towing at sea. They also work well if you have to open lots of boxes with fiberous tape.

I carry a plain edge for EDC, but use the serrated edge where it is best. The addition of a section on a longer blade makes sense if you might need it a lot without the time to sharpen it (survival knife on the water, I.e. SOG SEAL).

For full disclosure, the owner of Spyderco has been a good friend since before he made his first knife. I have been using a Triangle sharpener since the first run.

Respectfully,

Jerry